Spectral Discrimination using Wavelength-Shifting Fiber-Coupled Scintillation Detectors

ABSTRACT

Methods for discriminating among x-ray beams of distinct energy content. A first volume of scintillation medium converts energy of incident penetrating radiation into scintillation light which is extracted from a scintillation light extraction region by a plurality of optical waveguides that convert the scintillation light to light of a longer wavelength. An x-ray beam initially incident upon the first volume of scintillation medium and traversing the first volume is then incident on a second volume of scintillation medium. The first and second scintillation media may be separated by an absorber or one or more further volumes of scintillation medium, and may also have differential spectral sensitivities. Scintillation light from the first and second scintillation volumes is detected in respective detectors and processed to yield a measure of respective low energy and high-energy components of the incident x-ray beam.

The present application is a divisional application of U.S. Ser. No.13/758,189, filed Feb. 4, 2013 and now issued as U.S. Pat. No. ______,and, through that application, claims the priority of U.S. ProvisionalPatent Applications, Ser. Nos. 61/598,521, and 61/598,576, both filedFeb. 14, 2012, and U.S. Provisional Patent Application, Ser. No.61/607,066, filed Mar. 6, 2012. All of the aforesaid applications areincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to fiber-coupled scintillation detectorsand to methods of their manufacture, and to systems and methods of x-rayinspection employing fiber-coupled scintillation detectors for efficientdetection of x-rays.

BACKGROUND ART

Fiber-coupled scintillation detectors of radiation and particles havebeen employed over the course of the past 30 years. In some cases, thescintillator is pixelated, consisting of discrete scintillator elements,and in other cases, other stratagems are employed (such as orthogonallycrossed coupling fibers) in order to provide spatial resolution.Examples of fiber-coupled scintillation detectors are provided by U.S.Pat. No. 6,078,052 (to DiFilippo) and U.S. Pat. No. 7,326,9933 (toKatagiri et al.), both of which are incorporated herein by reference.Detectors described both by DiFilippo and Katagiri et al. employwavelength-shifting fibers (WSF) such that light reemitted by the corematerial of the fiber may be conducted, with low attenuation, tophoto-detectors disposed at a convenient location, often distant fromthe scintillator itself. Spatial resolution is of particular value inapplications such as neutron imaging. Spatial resolution is alsoparamount in the Fermi Large Area Space Telescope (formerly, GLAST)where a high-efficiency segmented scintillation detector employs WSFreadout for detection of high-energy cosmic rays, as described inMoiseev, et al., High efficiency plastic scintillator detector withwavelength-shifting fiber readout for the LAST Large Area Telescope,Nucl. Instr. Meth. Phys. Res. A, vol. 583, pp. 372-81 (2007), which isincorporated herein by reference.

Because of the contexts in which fiber-coupled scintillator detectorshave been employed to date, all known fiber-coupled scintillatordetectors have counted pulses produced by individual interactions ofparticles (photons or massive particles) with the scintillator, therebyallowing the energy deposited by the incident particle to be ascertainedbased on the cumulative flux of light reemitted by the scintillator.

The detection requirements of x-ray backscatter inspection systems,however, are entirely different from the requirements addressed byexisting fiber-coupled scintillation detectors. Backscatter x-rayinspection systems have been used for more than 25 years to detectorganic materials concealed inside baggage, cargo containers, invehicles, and on personnel. Because organic materials in bulkpreferentially scatter x-rays (by Compton scattering) rather than absorbthem, these materials appear as brighter objects in backscatter images.Insofar as incident x-rays are scattered into all directions,sensitivity far overrides spatial resolution as a requirement, and inmost scatter applications, detector spatial resolution is of no concernat all, since resolution is governed by the incident beam rather than bydetection.

The specialized detection requirements of large area and highsensitivity posed by x-ray scatter systems are particularly vexing inthe case of “conventional” scintillation detectors 100 of the type shownin a side cross-section in FIG. 1A and in a front cross-section in FIG.1B. An example of such a detector is described in U.S. Pat. No.5,302,817 (to Yokota), and is incorporated herein by reference.Typically, a light-tight box 102 is lined with scintillating screens 103where incident x-ray radiation 101 is converted to scintillation light,typically in the UV, visible, or longer wavelength, portions of theelectromagnetic (EM) spectrum. Large-photocathode-area photomultipliertubes (PMTs) 105 are coupled to receive scintillation light viaportholes 108. One problem lies in that a fraction of the scintillationlight originating within the screen is transmitted from the screen intothe enclosed volume. The remaining scintillation light is lost in thescreen material. Scintillating screens 103 are designed to maximize thefraction of emitted light, which is tantamount to ensuring a largetransmission coefficient T for the interface between screen 103 and themedium (typically air) filling the detector volume. However, in aconventional backscatter detector of the sort depicted in FIGS. 1A and1B, the scintillation screens 103 should also serve as good reflectorsbecause scintillation light, once emitted into the volume of box 102,typically needs multiple reflections until it reaches a photo-detector105. So, the reflection coefficient R of the screen surface should alsobe large, however, since the sum of T and R is constrained to be unity,both T and R cannot be maximized simultaneously, and a compromise mustbe struck. As a result, the light collection efficiency of theconventional backscatter detector is inherently low, with only a fewpercent of the generated scintillation light collected into the photodetectors.

For an imaging detector, the photon statistical noise is calculated interms of the photons absorbed by the detector and used to generate theimage. Any photons which pass through the detector without beingabsorbed, or even those that are absorbed without generating imageinformation, are wasted and do not contribute to reducing noise in theimage. Since photons cannot be subdivided, they represent thefundamental quantum level of a system. It is common practice tocalculate the statistical noise in terms of the smallest number ofquanta used to represent the image anywhere along the imaging chain. Thepoint along the imaging chain where the fewest number of quanta are usedto represent the image is called a “quantum sink”. The noise level atthe quantum sink determines the noise limit of the imaging system.Without increasing the number of information carriers (i.e., quanta) atthe quantum sink, the system noise limit cannot be improved. Poor lightcollection can possibly create a secondary quantum sink, which is to saythat it will limit the fraction of incident x-rays resulting in PMTcurrent. Moreover, it will increase image noise. Light collectionefficiency can be improved by increasing the sensitive area of thephoto-detectors, however, that path to efficiency is costly.

The structure of scintillating screen typically employed in prior artx-ray scintillation detectors is now described with reference to FIG. 2.A layer of composite scintillator 202 is sandwiched between a backersheet 204 for structural support and a thin, transparent protective film206 composed of polyester, for example. The composite scintillatorconsists of typically micron-size inorganic crystals in an organicmatrix or resin. The crystals are the actual scintillating material.Barium fluoro-chloride (BaFCl, or “BFC”) or gadolinium oxysulfide(Gd₂O₂S, or “Gadox”) doped with rare earth elements are common choicesfor these. The stopping power of the screen is determined by thethickness of the composite scintillator layer 202, which is typicallymeasured in milligrams of scintillator crystal per unit area. Becausethe inorganic scintillators (such as BFC or Gadox) suffer from highself-absorption, the composite scintillator layer has to be kept ratherthin in order to extract a good fraction of the scintillation light.This limits the useful stopping power of the screen and makes itsuitable only for the detection of x-rays with energies up to around 100keV.

Therefore, it would be advantageous to have a scintillation detector forx-ray scatter detection applications that provides for more efficientextraction, collection, and detection of scintillation light.

As briefly discussed at the outset above, wavelength-shifting fibers(WSF) have long been employed for scintillation detection. Wavelengthshifting fibers consist of a core with relatively high refractive index,surrounded by one or more cladding layers of lower refractive index. Thecore contains wavelength-shifting material, also referred as dye.Scintillation light which enters the fiber is absorbed by the dye which,in turn, emits light with a longer wavelength. The longer wavelengthlight is emitted isotropically in the fiber material. Total internalreflection traps a fraction of that light and conducts it over longdistances with relatively low loss. This is possible, as described withreference to FIG. 3, because the absorption 304 and emission 302wavelength ranges of the dye effectively do not overlap so that thewavelength-shifted light is not reabsorbed. The captured fraction isdetermined by the ratio of the refractive indices at surfaces of thefiber. An additional advantage of WSF is that the wavelength shiftingcan bring the scintillation light 306 into the sensitive wavelengthrange of the photo detector (PMT, silicon photomultiplier (SiPM), orMultiple-Pixel Photon-Counter (MPPC), or otherwise).

Scintillator structures have been produced using many manufacturingtechnologies, including, for example, die-casting, injection molding (asdescribed by Yoshimura et al., Plastic scintillator produced by theinjection-molding technique, Nucl. Instr. Meth. Phys. Res. A, vol. 406,pp. 435-41 (1998), and extrusion, (as described in U.S. Pat. No.7,067,079, to Bross, et al.), both of which references are incorporatedherein by reference.

SUMMARY OF EMBODIMENTS OF THE INVENTION

In accordance with various embodiments of the present invention, systemsand methods are provided that apply fiber-coupled scintillationdetectors to problems in backscatter and transmission x-ray inspection.

For convenience of notation, a wavelength-shifted fiber-coupledscintillation detector may be referred to herein as an “Sc-WSF”detector.

In a first embodiment of the present invention, a detector ofpenetrating radiation is provided that has an unpixelated volume ofscintillation medium for converting energy of incident penetratingradiation into scintillation light. The detector has multiple opticalwaveguides, aligned substantially parallel to each other over ascintillation light extraction region that is contiguous with theunpixelated volume of the scintillation medium, The optical waveguidesguide light derived from the scintillation light to a photo-detector fordetecting photons guided by the waveguides and for generating a detectorsignal.

In other embodiments of the present invention, the detector may alsohave an integrating circuit for integrating the detector signal over aspecified duration of time.

In an alternate embodiment of the invention, a detector of penetratingradiation is provided that has a volume of scintillation medium forconverting energy of incident penetrating radiation into scintillationlight and a plurality of optical waveguides, aligned substantiallyparallel to each other over a scintillation light extraction regioncontiguous with the volume of the scintillation medium. The opticalwaveguides guide light derived from the scintillation light to aphoto-detector that generates a detector signal. Finally, an integratingcircuit for integrating the detector signal over a specified duration oftime.

In further embodiments of the invention, the optical waveguides in theforegoing detectors may be adapted for wavelength shifting of thescintillation light and, more particularly, may be wavelength-shiftingoptical fibers. The scintillation medium may include a lanthanide-dopedbarium mixed halide such as barium fluorochloride. The photo-detectormay include a photomultiplier.

In yet further embodiments of the invention, the square of the thicknessof any of the foregoing detectors, divided by the area of the detector,may be less than 0.001. At least one of the plurality of waveguides maylacks cladding and the scintillation medium may be characterized by anindex of refraction of lower value than an index of refractioncharacterizing the waveguide. The optical waveguides may be disposed inmultiple parallel planes, each of the parallel planes containing asubset of the plurality of optical waveguides.

In other embodiments of the invention, the detector may have a pluralityof layers of scintillator medium successively encountered by an incidentbeam, and the layers may be characterized by distinct spectralsensitivities to the incident beam. Alternating layers of scintillatormay include Li⁶F:ZnS(Ag) alternating with at least one of fiber-coupledBaFCl(Eu) and fiber-coupled BaFI(Eu). A first of the plurality of layersof scintillator medium may be a wavelength-shifting fiber-coupleddetector preferentially sensitive to lower-energy x-rays, and a last ofthe plurality of layers of scintillator medium may be a plasticscintillator.

Segments of scintillator medium may be disposed in a plane transverse toa propagation direction of an incident beam, and may be distinctlycoupled to photo-detectors via optical fibers.

In accordance with another aspect of the present invention, a method formanufacturing a scintillation detector, the method comprising extrudinga shell of scintillating material around an optical waveguide, and, in aparticular embodiment, the optical waveguide is a wavelength-shiftingoptical fiber.

In an alternate embodiment, a method for detecting scattered x-rayradiation has steps of:

-   -   a. providing a detector characterized by a plurality of        individually read-out segments; and    -   b. summing a signal from a subset of the individually read-out        segments, wherein the subset is selected on a basis of relative        signal-to-noise.

In another aspect of the invention, a method is provided for detectingscattered x-ray radiation. The method has steps of:

-   -   a. providing a detector characterized by a plurality of        individually read-out segments; and    -   b. summing a signal from a subset of the individually read-out        segments, wherein the subset is selected on a basis of a known        position of a primary illuminating beam.

A mobile x-ray inspection system is provided in accordance with anotherembodiment. The inspection system has a source of x-ray radiationdisposed upon a conveyance having a platform and ground-contactingmembers, and a fiber-coupled scintillation detector deployed outside theconveyance during inspection operation for detecting x-rays that haveinteracted with the inspected object.

The mobile x-ray inspection system may also have a fiber-coupledscintillation awning detector deployed above the inspected object duringa course of inspection, and the awning detector may slide out from aroof of the conveyance prior to inspection operation. There may also bea skirt detector deployed beneath the platform of the conveyance, and aroof detector for detection of spaces higher than the conveyance, aswell as substantially horizontal and substantially upright fiber-coupledscintillator detector segments. The substantially horizontal andsubstantially upright fiber-coupled scintillator detector segments maybe formed into an integral structure.

In accordance with another aspect of the present invention, an apparatusis provided for detecting radiation incident upon the apparatus, theapparatus comprising:

-   -   a. a plurality of substantially parallel active collimation        vanes comprising wavelength-shifted fiber-coupled scintillation        detectors sensitive to the radiation for generating at least a        first detection signal;    -   b. a rear broad area detector for detecting radiation that        passes between substantially parallel active collimation vanes        of the plurality of active collimator vanes and generating a        second detection signal; and    -   c. a processor for receiving and processing the first and second        detection signals.

In accordance with an alternate embodiment of the invention, a top-downimaging inspection system is provided for inspecting an object disposedon an underlying surface. The top-down imaging inspection system has asource of substantially downward pointing x-rays and a linear detectorarray disposed within a protrusion above the underlying surface. Thelinear detector array may include wavelength-shifted fiber-coupledscintillation detectors.

In accordance with another aspect of the invention, an x-ray inspectionsystem is provided for inspecting an underside of a vehicle. The x-rayinspection system has a source of substantially upward pointing x-rayscoupled to a chassis and a wavelength-shifting fiber-coupledscintillator detector disposed on the chassis for detecting x-raysscattered by the vehicle and by objects concealed under or within thevehicle. The chassis may be adapted to be maneuvered under the vehicleby at least one of motor and manual control.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing features of the invention will be more readily understoodby reference to the following detailed description, taken with referenceto the accompanying figures, in which:

FIGS. 1A and 1B show side and front cross-sectional views, respectively,of a “box-type” prior art scintillation detector.

FIG. 2 is a schematic view of a prior art scintillator screen.

FIG. 3 depicts spectral relationships among scintillation light andtypical wavelength-shifting fiber absorption and emission spectra.

FIG. 4 is a perspective schematic view of an array ofwavelength-shifting fibers sandwiched between scintillator material, inaccordance with an embodiment of the present invention.

FIG. 5 is a cross-sectional schematic view of an array ofwavelength-shifting fibers embedded within a matrix of scintillatormaterial, in accordance with an embodiment of the present invention.

FIG. 6A is a perspective view of a cylindrical scintillator extrudedabout a WSF, in accordance with an embodiment of the present invention.

FIG. 6B is a schematic depiction of a system for extruding a cylindricalscintillator about a WSF, in accordance with an embodiment of thepresent invention.

FIG. 6C is a cross-sectional view of an extruder for co-extruding acylindrical scintillator with a WSF, in accordance with an embodiment ofthe present invention.

FIG. 7 is a schematic cross-section of a scintillation detector withmultiple rows of WSF, in accordance with an embodiment of the presentinvention.

FIG. 8 is a top view of a wavelength-shifted fiber-coupled scintillationdetector in accordance with an embodiment of the present invention.

FIG. 9 shows roof and skirt backscatter detectors, stowed in accordancewith embodiments of the present invention, while

FIG. 10 shows the same detectors deployed during the course ofinspection operations.

FIG. 11 shows an awning detector and a skirt detector for use with abackscatter inspection system in accordance with embodiments of thepresent invention.

FIG. 12 is a cross-sectional schematic view of a stack of scintillatorlayers for use as a high-energy x-ray transmission detector, inaccordance with an embodiment of the present invention.

FIGS. 13A and 13B show a layered transmission detector inside a2-inch-high speed bump, in accordance with an embodiment of the presentinvention, while FIG. 13C shows a cross section of the detector assemblyinserted into the speed bump frame.

FIG. 14A shows a perspective view of a segmented x-ray transmissiondetector for measurement of the distribution of detected intensityacross the width of an x-ray beam, in accordance with an embodiment ofthe present invention, while FIGS. 14B and 14C show an end-oncross-section and a typical beam profile of the detector of FIG. 14A.

FIG. 15 is a cross-sectional view of a scintillation detector withmulti-energy resolution, in accordance with an embodiment of the presentinvention.

FIG. 16 shows a multi-layer scintillation detector for detection of bothx-rays and thermal neutrons, in accordance with an embodiment of thepresent invention.

FIG. 17 shows a perspective view of a detector with active collimators.

FIGS. 18A and 18B show perspective and cross-sectional views of aWSF-detector used as an active collimator in accordance with anembodiment of the present invention, and FIGS. 18C and 18D show anarrangement with independent readouts separated by a light-tight x-rayabsorber to distinguish radiation striking each face, in accordance witha further embodiment of the present invention.

FIGS. 19A and 19B shows multiple detectors folding out of a hand-heldscanner, in stored and deployed conditions, respectively, in accordancewith an embodiment of the present invention.

FIGS. 20A and 20B show a backscatter unit that, by virtue of Sc-WSFdetectors in accordance with the present invention, may be slid under avehicle for under-chassis inspection.

FIGS. 21A and 21B depict the use of a right-angled combination ofdetectors based on Sc-WSF technology in conjunction with a mobileinspection system and in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In accordance with embodiments of the present invention, the opticalcoupling of scintillator material to optical waveguides, and, moreparticularly, to wavelength-shifting fibers, advantageously enablesobjectives including those peculiar to the demands of x-ray scatterdetection.

Definitions:

The term “image” shall refer to any unidimensional or multidimensionalrepresentation, whether in tangible or otherwise perceptible form, orotherwise, whereby a value of some characteristic (such as fractionaltransmitted intensity through a column of an inspected object traversedby an incident beam, in the case of x-ray transmission imaging) isassociated with each of a plurality of locations (or, vectors in aEuclidean space, typically

²) corresponding to dimensional coordinates of an object in physicalspace, though not necessarily mapped one-to-one thereonto. An image maycomprise an array of numbers in a computer memory or holographic medium.Similarly, “imaging” refers to the rendering of a stated physicalcharacteristic in terms of one or more images.

Terms of spatial relation, such as “above,” “below,” “upper,” “lower,”and the like, may be used herein for ease of description to describe therelationship of one element to another as shown in the figures. It willbe understood that such terms of spatial relation are intended toencompass different orientations of the apparatus in use or operation inaddition to the orientation described and/or depicted in the figures.

Where an element is described as being “on,” “connected to,” or “coupledto” another element, it may be directly on, connected or coupled to theother element, or, alternatively, one or more intervening elements maybe present, unless otherwise specified.

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting. The singular forms “a,”“an,” and “the,” are intended to include the plural forms as well.

WSF Detectors

Referring, first, to FIG. 4, in one embodiment of the invention, a layerof closely spaced parallel wavelength-shifting fibers 400 is sandwichedbetween two layers 403 of composite scintillating screen. The preferredscintillator material is europium-doped barium fluorochloride(BaFCl:Eu), although other scintillators, such as BaFI:Eu, or otherlanthanide-doped barium mixed halides (including, by way of furtherexample, BaBrI:Eu and BaCsI:Eu), may be used within the scope of thepresent invention. Since scintillator materials employed for x-raydetection typically exhibit very strong self-absorption of scintillationphotons, embodiments in accordance with the present inventionadvantageously allow unusually large volumes of scintillator 403 to beemployed while still efficiently coupling out scintillation signal.

One advantage to using composite scintillation screen in the presentapplication is that it allows for fabrication by extrusion of afiber-coupled scintillation detector.

Composite scintillator 403 is structurally supported by exterior layers404 of plastic, or other material, providing mechanical support. Opticalcontact between the fiber cladding 401 and the composite scintillator403 is established by filling the voids with index-matching material 405of suitable refractive index which is transparent to the scintillationlight. The refractive index of the filling material is chosen tooptimize the collection of primary light photons into the WSF and thecapture of wavelength-shifted photons in the fiber. Filling material 405may be optical grease or optical epoxy, for example, though any materialis within the scope of the present invention.

Upon incidence of x-ray photons, scintillation light emitted byscintillator 403 is coupled via cladding 401 into core 407 of therespective fibers, down-shifted in frequency (i.e., red-shifted) andpropagated to one or more photo-detectors 805 (shown in FIG. 8, forexample). Light from the fiber cores 407 is converted into a current viaphoto-detector 805, and the current is integrated for an interval oftime, typically in the range of 1-12 μs, to obtain the signal strengthfor each pixel. Integration of the detector signal may be performed byan integrating circuit (not shown), such as an integratingpre-amplifier, for example.

Referring now to FIG. 5, wavelength-shifting fibers 400 are embedded inthe matrix of the scintillating screen 503. Embedding the WSF into thescintillating medium creates the best optical contact.

In yet another embodiment of the invention, described now with referenceto FIG. 6A, composite scintillator material 603 is applied like acladding or shell around a WSF 601 with core 602. This application lendsitself to an extrusion-style manufacturing process and allows making themost effective use of costly scintillator material 603. The scintillatormaterial 603 is sealed off with a protective layer 604 which also actsas a reflector to the scintillation light. Within the scope of thepresent invention, the cladding may be omitted when the scintillator hasa lower index of refraction than the fiber and the scintillator-fiberbond has the necessary smoothness and robustness.

A wavelength-shifting polymer optical fiber may be manufactured, inaccordance with an embodiment of the invention now described withreference to the system schematic depicted in FIG. 6B. Sources of WSFpolymer melt 606, low-refractive-index cladding polymer melt 608, andphosphor-embedded optically transparent polymer melt 610, all underpressure, are fed into a co-extrusion die 612 within extrusion zone 614,and co-extruded. Dry gas 611, such as dry air or nitrogen, for example,is sprayed onto the extruded fiber for cooling. Polymer melt with alight-reflective pigment (such as TiO₂, for example) 616 is fed underpressure into an extrusion die 618 for a light-reflective jacket overthe scintillator-coated WSF 613. The resultant scintillator-loaded WSF620 is wound for storage by winder 622. FIG. 6C shows a cross-sectionalview of a co-extrusion system, for use in accordance with embodiments ofthe present invention, for the manufacture of scintillator-coated WSF.The WSF polymer melt 606 is injected, along with thelow-refractive-index cladding polymer melt 608 and phosphor-embeddedoptically transparent polymer melt 610, into co-extrusion die 612.Polymer melt with a light-reflective pigment 616 is fed under pressureinto extrusion die 618. The completed fiber has a WSF core 602, alow-index cladding 601, a scintillator-loaded cladding 603, and areflective coating 604.

For all embodiments of a scintillation detector in accordance with thepresent invention, it is advantageous that the thickness of thescintillator material be optimized for the energy of the radiation to bedetected. The design should ensure sufficient light collection to avoida secondary quantum sink. In particular, embodiments of the inventiondescribed herein provide for detectors of extraordinary thinnessrelative to their area.

Definitions: For purposes of the present description, and in anyappended claims, the term “thickness,” as applied to a scintillationdetector, shall represent the mean extent of the detector in a dimensionalong, or parallel to, a centroid of the field of view of the detector.The term area, as applied to a detector, or, equivalently, the term“active area” shall refer to the size of the detector measured in aplane transverse to centroid of all propagation vectors of radiationwithin the field of view of the detector.

Embodiments of the present invention, even those with as many as 8 WSFlayers, have ratios of the square of detector thickness to the activedetector area that are less than 0.001. For example, an 8-layer detectorwith an area of 48″×12″ has a thickness no greater than 0.5″, such thatthe ratio of the square of the thickness to the detector area is 0.0005.This thickness-squared-to-area ratio is typically an order of magnitude,or more, smaller than the comparable ratio for backscatter detectorswhere scintillator light is directly detected by a photo-detector.

In accordance with a further embodiment of the invention depicted inFIG. 7, the useful stopping power of the detector can be increased bycombining multiple layers 701, 702 of WSF 400 (or other opticalwaveguides thereby increasing the depth of scintillator material 403along the path of the incident radiation.

An embodiment of a wavelength-shifted scintillator detector inaccordance with the present invention is shown in FIG. 8.Wavelength-shifting fibers 801 are embedded within scintillator material803, coupling light, and downshifting it in frequency for detection byphotomultiplier tubes 805.

In accordance with various of the embodiments heretofore described, theends of the WSF are bundled and optically coupled to at least one photodetector. Examples of suitable photo detectors include PMTs and siliconphotomultipliers (SiPMs).

Advantages of the detector, the invention of which is described herein,include the efficiency of detection, and the low geometrical profile ofimplementation. This allows greater freedom in designing a detectionsystem and it makes entirely new, space constrained applicationspossible. The mechanical flexibility of the detector structure allowsshaping the detector surface to conform to the application, such as animplementation in which an imaged object is surrounded by detectorvolume. The low profile also makes it relatively easy to orient andshield the detector area in ways to minimize the detection of unwantedscatter radiation (crosstalk) from a nearby x-ray imaging system.

The extraction of scintillation light over a large region ofscintillator enables detectors of large width-to-depth aspect ratio. Inparticular, detectors subtending spatial angles of 0.1 sr, of more, arefacilitated by embodiments of the present invention.

In a typical backscatter x-ray imaging system, an x-ray pencil beamscans an imaged target in a linear motion, while elongated radiationdetectors are arranged on both sides of an exit aperture of an x-raysource. As the pencil beam moves, the detector area closest to the beamwill typically receive the strongest signal and detector area furtherfrom the beam less. If the detector area is segmented into individuallyreadable sections the signal to noise ratio of the detection system canbe improved by only reading the segments with a good signal to noiseratio and neglecting the segments which would contribute predominantlynoise to the summed signal. The selection of contributing detectorsegments can be made based on the actually detected signal or based onthe known position of the pencil beam.

Advantages of Scintillator Fabrication by Extrusion

The extrusion, or “automated coating” process, described above withreference to FIGS. 6A-6C, is in stark contrast to typical methods oflaying down polycrystalline scintillation material, such as BaFCl(Eu),on a flat backing. The extrusion method of fabricating individualwavelength-shifting fibers coated with a uniform thickness ofscintillator, as taught above, produces fibers that can be contoured sothat the restrictions on the shape of a Sc-WSF detector is governedprimarily by the requirement of full capture in the fiber by totalinternal reflection. The concept of uniformly coated coupling fibersgives greater freedom to the design of backscatter (BX) detectors,especially hand-held and robot-mounted detectors, where space is at apremium.

Deployable Detectors to Increase Geometric Efficiency of ScatteredX-Rays

Some mobile x-ray systems, such as those described, for example, in U.S.Pat No. 5,764,683, to Swift, et al. and U.S. Pat. No. 7,099,434, toChalmers et al., both of which are incorporated herein by reference, usethe method of backscattered x-rays (BX) to inspect cars and trucks fromone side. The former uses detectors deployed outside a conveyance duringoperation, whereas the latter uses a detector area entirely containedwithin an enclosure, namely the skin of a conveyance. Both uselarge-area detectors to maximize the efficiency of detecting thescattered x-rays. The areal backscatter detector coverage in the case ofa product in accordance with the teachings of the Chalmers '434 Patentcovers on the order of 20 square feet of the interior surface of anenclosure that faces the target. This covert detector area hasrelatively poor geometrical efficiency for collecting the scatteredradiation from high or low targets. The intrinsically deep geometricalprofile of such detectors, necessary for direct capture of thescintillation light by photomultipliers, is inimical to deploymentoutside the van.

Definitions: As used herein, and in any appended claims, the term“large-area detector” shall refer to any single detector, or to anydetector module, subtending an opening angle of at least 30° in each oftwo orthogonal transverse directions as viewed from a point on an objectundergoing inspection, equivalently, characterized by a spatial angle ofat least π steradians.

A “conveyance” shall be any device characterized by a platform borne onground-contacting members such as wheels, tracks, treads, skids, etc.,used for transporting equipment from one location to another.

An Sc-WSF detector, in accordance with embodiments of the presentinvention, makes practical the unobtrusive storage of large-areadetectors that can be quickly deployed outside the van in positions thatsubstantially enhance detection efficiency.

Referring, now, to FIG. 9, a large-area Sc-WSF awning detector 1101 isshown in a stowed position, stored on the roof of a backscatterinspection van 1103, and a thin skirt detector 1105 is shown in a stowedposition above a wheel of the backscatter inspection van. In FIG. 10,both the roof and skirt detectors are shown as deployed to increase thesolid angle for detecting higher and lower targets, respectively; theawning detector is deployed above an inspected object during the courseof inspection, while the skirt detector is deployed, at least in part,beneath the platform of the conveyance. In another embodiment of theinvention, described with reference to FIG. 11, an awning detector 1301may be deployed for low, close targets, such as for detection ofcontraband in the trunk or far side of a car 1303. Awning detector 1301may slide out from a roof of the conveyance prior to inspectionoperation. FIG. 11 also shows the deployment of Sc-WSF skirt detectors1105 used to efficiently examine the tires, wheel wells, and theinterior of close vehicles.

Scanning pencil beams of x-rays not only reveal interior objects byanalyzing the backscattered radiation but, in some applications, canobtain additional information by the simultaneous analysis oftransmission (TX) and forward scattered (FX) radiation. The TX and FXdetectors need not be segmented since the cross-sectional area of thepencil beam, together with the integration time of the signal, definesthe pixel size. Moreover, the TX and FX detectors only need to be totalenergy detectors since, in most applications, the flux of the TX or FXx-rays is too high for pulse counting. Scintillation screens are thetraditional detectors for such scanning beam applications. Sc-WSFdetectors substantially extend the range of applications of present TXand FX scintillation detectors, as the following examples make clear.

TX for X-Ray Beams up to at Least 250 keV

The absorption efficiency of traditional scintillation screens, made,for example, of BaFCl(Eu) or Gadox, drops below 50% for x-ray energiesabove ˜80 keV. The 50% point for two layers is about 100 keV. By way ofdistinction, Sc-WSF detector can be made with more than two layers ofscintillators without substantially increasing the profile of thedetector. A cost-effective Sc-WSF detector, with 4 layers, can be usedfor TX with scanning x-ray beams generated by a standard 140 keV x-raytube. A multi-layer detector such as the 9-layer detector, as shown inFIG. 12, and designated there generally by numeral 1400, can be highlyeffective for a detecting x-rays 1402 emitted by a standard 225 keVx-ray tube (not shown), such as that used in the x-ray inspection ofvehicles through portals. Layers 1404 of scintillator material areshown, and WSF fibers 1406 coupled to photo-detectors 1408.

Transportable TX Detector for Top-Down Imager in Three-Sided PortalInspection

The thin profile of the multi-layer transmission (TX) detector makespractical a top-of-the-road transmission (TX) detector. FIGS. 13A and13B show such a detector inside a 2-inch-high speed bump 1131 strongenough to support a fully-loaded tractor trailer, and requiring noexcavation of the ground for deployment. Source 1132 of penetratingradiation emits fan beam 1134 incident upon a linear detector assembly1135 within frame 1136 of speed bump 1131 or a similar protrusion abovean underlying surface. Detector assembly 1135 includes segments ofscintillator material 1137 separated by vanes 1138 of high atomicnumber. As described above, for example with reference to FIG. 4,scintillation light is coupled to photo-detectors by means ofwave-length shifting optical fibers 1139.

Segmented TX Detector for Determining the Scan Beam Intensity Profile

Referring now to FIGS. 14A and 14B, a segmented transmission detector,designated generally by numeral 1141, is shown for measuring a scan beamintensity profile of incident x-rays 1143. Alignment of the Sc-WSFdetector 1141 (used in transmission) with the plane of a scanning pencilbeam presents a significant challenge when the TX detector is deployedfor a mobile security system. FIG. 14B shows a cross section of avertical Sc-WSF detector 1141 (otherwise referred to herein, whenappropriate, as a “transmission detector” or “TX detector”) withindependent read-out of the fibers 1145 of the WSFs, provides the meansto simultaneously measure both the transmitted intensity of each pixeland the linear distribution across the beam width to determine itscentroid position. Fibers 1145 are routed in bundles 1147 to individualphoto-detectors 1149 such as PMTs. The distribution of the intensity canextend out to obtain the forward scattered intensity, which containsuseful information as to the scattering material, and gives a measure ofthe in-scattered radiation that is being counted as Transmissionintensity.

The relative position of the detector plane and the plane of scanningx-rays can be controlled automatically. The detector for this concept isshown schematically in FIG. 14A. A reflecting surface 1148 may beprovided at an end of detector 1141 distal to photo-detectors 1149.

With a single data channel for a transmission signal, the spatialresolution along the traffic direction (transverse to a fan-shapedilluminating x-ray beam) is determined by the smaller of the followingtwo dimensions: the width of the sensitive detector area or the beamsize across the TX detector. (For heuristic purposes, the case ofundersampling is not considered in this description.) Spatial resolutionmay be improved, however, by narrowing the sensitive detector area, asnow described with reference to FIG. 14C. In accordance with embodimentsof the present invention, the spatial resolution across the direction oftraffic (along the detector line) is enhanced by employing multipledetectors of a detector array 1450 associated with a plurality ofchannels (A, B, C, in FIG. 14C) and interlacing their sensitive areas.The pitch of the interlace pattern depends on the beam width along thedetector. Ideally the pitch (i.e., the spacing between two detectors1451 and 1454 associated with a single channel “A”) has to be largeenough so that two detector segments of the same detection channel donot receive direct radiation from the beam at the same time. The beamintensity profile is depicted by numeral 1456. For practical purposesthe requirement is not as stringent, since some amount of crosstalkbetween pixels is acceptable. The multiple, resulting images need to beinterlaced, employing any method, including methods well-known in theart, to create one higher-resolution image. It should be noted thatimprovement of the spatial resolution at the detector comes at theexpense of flux and is, thus, limited by signal-to-noise considerations.

Another configuration within the scope of the present invention includea combination of the vertical detector 1141 shown in FIG. 14A withhorizontal road detector 1135 of FIG. 13B to form an L-shaped detectorthat is advantageously easily set up and aligned.

In yet another embodiment of the invention, a transmission detectorarray 1450 (regardless of geometrical orientation, whether vertical,horizontal, L-shaped, etc.) is segmented into a plurality of units; suchas B, C and A of FIG. 14C. As shown, the beam profile 1456 is symmetricwith respect to B and A so that the ratio of the measured intensities isunity. If, for any reason, the alignment changes, that ratio changesdramatically. If the alignment skews as an illuminating x-ray pencilbeam scans up and down, the change in the ratio of B/A measures the boththe skewness and the lateral shift. Collected data can then be correctedfor such a shift on a line-by-line basis.

Dual-Energy and Multi-Energy TX Detectors for Material Identification

Separating the signals from front and back layers of scintillatorsallows the front layer to give a measure of the low-energy component ofeach pixel while the back layer gives a measure of the high-energycomponents. Putting a layer of absorbing material between the front andback scintillators is a standard way to enhance the difference betweenlow and high energy components, and that is easily done with a Sc-WSFdetector.

The Sc-WSF detector makes practical a dual-energy detector consisting ofa layer of Sc-WSF, such as BaFCl-WSF, on top of a plastic scintillatordetector; the BaFCl is sensitive to the low-energy x-rays and not thehigh-energy x-rays, while the plastic detector is sensitive to thehigh-energy x-rays and very insensitive to low energy x-rays.

An alternative and potentially more effective material discriminator canbe made by using more than two independent layers of Sc-WSF, withseparate readouts for each layer. A passive absorber, such as anappropriate thickness of copper, can be inserted after the top Sc-WSF toenhance dual energy application, as is practiced with segmenteddetectors. Alternatively, the middle scintillator can be used as anactive absorbing layer. The measurement of three independent parametersallows one to get a measure of both the average atomic number of thetraversed materials and the extent of beam hardening as well. The Sc-WSFcan be further extended to obtain more than three energy values for eachpixel, the limit being the statistical uncertainties, which increasewith the number of components. Detector 1400 shown in FIG. 12 is anextreme example of such a detector.

An important application of Dual-Energy TX is for x-ray personnelscanners at airport terminals. Providing TX images simultaneously withBX has proved useful for inspection. Adding dual-energy to the TX imageshas hitherto been impractical primarily because of size constraintsimposed by conventional detectors. Sc-WSF eliminates those constraintsand promises to significantly improve performance, since multipledetectors, with distinct energy sensitivities, may be stacked, as shownin FIG. 15, where a dual- (or multi-) energy detector 1500 includes anSc-WSF detector 1508, sensitive to a lower energy component of incidentx-rays 1501, positioned in front of a slab of plastic scintillator 1502,which is sensitive to the higher energy x-rays. Sc-WSF detector 1508contains a scintillator 1504 read out by two layers of WS fibers 1506.

Compact Radiation Detector of Gamma and Neutron Radiation

The Sc-WSF method makes practical a small, lightweight, inexpensive,monitor of neutrons and gamma rays 1601. BaFCl(Eu)-WSF is quitesensitive to gamma radiation while being insensitive to neutrons, whileLi⁶F:ZnS(Ag)-WSF is insensitive to gamma rays and quite sensitive todetecting thermal neutrons. FIG. 16 shows a multi-layer “Dagwood”sandwich consisting of one or more layers 1602 of BaFCl(Eu), read out bya single photo-detector (not shown) via optical fibers 1604, and one ormore layers 1606 of Li⁶F:ZnS(Ag)-WSF, read out by a second, independent,photo-detector (not shown), with the active elements occupying athickness of no more than one or two centimeters. An appropriate layerof neutron moderator 1612, such as polyethylene, may be placed on eitherside of the Li⁶F:ZnS(Ag)-WSF to enhance the efficiency for detectingneutrons. Optically reflective foil 1608, such as aluminum foil,confines scintillation to respective detector regions.

U.S. patent application, Ser. No. 13/163,854 (to Rothschild), entitled“Detector with Active Collimators,” and incorporated herein byreference, describes a backscatter detector module 30 that increases thedepth of inspection by distinguishing scatter from the near and farfield of inspected objects, as depicted in FIG. 17. The angle of a setof active collimating vanes 31 may either be adjusted once at thefactory, or may be attached to any kind of electro-mechanical deviceprovided to dynamically adjust them, depending on the type and/ordistance of the object being scanned. The scintillation light from thecollimating vanes is detected by one or more photo-detectors (forexample, by PMTs 32 located at the top and bottom of the frontcompartment of the detector). A rear compartment 36 of the detector isoptically isolated from a front compartment 35 by a light baffle 34, andscintillation light from x-rays detected in rear compartment 36 arecollected by a second set of one or more photo-detectors (for example,PMTs 37 mounted on the rear face of the detector. The rear compartmentmay be lined with scintillating phosphor screen, for example, or, inother embodiments of the invention, may contain plastic or liquidscintillator.

A useful addition to a standard backscatter unit would be a “venetianblind” collimator made of scintillator. The slats intercept radiationthat does not enter directly through the gaps between the slats so thatthe box detectors preferentially detect deeper interior objects. Theactive collimators record the rejected radiation. The light from theactive collimators is detected by PMTs, whose collection efficiencydecreases rapidly as the gap between collimators decrease. Replacing thePMTs and scintillator vanes with vanes consisting of Sc-WSF detectorssolves major shortcomings and makes venetian-blind collimatorspractical. First, light collection is independent of the gap widthbetween vanes. Second, the active area of the PMTs or siliconphotomultipiers used to collect the light from the active collimators isgenerally much smaller than the active area of needed PMTs, so that thecost of the photo-detectors is less. Third, the placement of thephoto-detector at the end of the WSF bundles is not critical to theefficiency of the light collection. Fourth, the signals from the WSFsfrom each slat can be processed independently, giving considerable scopefor maximizing the information about the interior of the inspectedobject. Fifth, the light from the thin scintillator screens on the frontand back of each vane can be collected by independent WSFs, which cansignificantly improve the depth discrimination.

FIGS. 18C and 18D depict (in perspective and in cross section,respectively) an active WSF collimator 181 sensitive to x-rays impingingfrom either side of the scintillator. Scintillation light from bothscintillator regions 182 is coupled to photo-detectors via waveshiftingoptical fibers 183. FIGS. 18A and 18B show (in perspective and in crosssection, respectively) an active WSF collimator 185 with independentreadouts 187 separated by a light-tight x-ray absorber 189 todistinguish radiation striking each face. For example, each collimator185 may consist, in one embodiment, of two layers of Sc-WSF detectors182, each containing an areal density of 60 mg BaFCl:Eu per cm2. Thelight-tight x-ray absorber 189 may consist of a thin layer of tin, whichalso provides structural support.

Detectors for Mini-Backscatter Inspection Systems

The thinness of Sc-WSF detectors provides a unique potential forapplications in which low weight and power are drivers. Referring toFIGS. 19A and 19B, a hand-held imaging system 193 is an example of suchan application. The power requirements, inspection time, and, quality ofthe image, are all affected by the solid angle of detection. Atraditional detector with, for example, a cross-section of 10 cm×10 cm(100 cm²), weighs about a half a kilogram. A 10-cm cube of Sc-WSF,weighing no more than twice as much, can be made of individual Sc-WSF 10cm×10 cm detectors, each less than 5 mm thick, that can be unfolded topresent a backscatter detection area of at least 2,000 cm², atwenty-fold increase in this example. The additional detection coveragecan make an order of magnitude improvement in the hand-held system'sperformance.

The thin profile of Sc-WSF detectors described herein provide forfitting contoured detectors into tight spaces. For example, detectorsmay be adapted for personnel scanners constrained to fit intoconstricted airport inspection spaces.

FIG. 19 shows an example in which four detectors 191 fold or slide outof hand-held scanner 193 to substantially increase the detectionefficiency, especially for items concealed deeper in the object beinginspected. Backscatter detectors 195 straddle irradiating beam 197.

Back-Sscatter Inspection of the Underside of Stationary Vehicles

The inspection of the underside of vehicles by a portable x-raybackscattering system presents special problems. The road clearance ofcars is not more than 8″ and can be as little as 6″. Fixed inspectionsystems, such as portals, can place a detector in the ground, or, asdescribed above, can be placed on the ground using Sc-WSF. Mobileunder-vehicle inspection systems, however, which are needed for securityin many areas, have never been developed. Inspectors rely on passiveinspection tools such as mirrors and cameras, which miss contraband inthe gas tank or are camouflaged to appear innocuous.

The Sc-WSF detectors make practical an x-ray backscatter system that isnot more than 6″ high. A sketch of a practical system is now describedwith reference to FIGS. 20A and 20B. The x-ray source consists of anelectromagnetic scanner 221 of an electron beam across an anode.Electromagnetic scanner 221 is driven by electronics module 223. Thex-rays are collimated by a linear array of apertures 225 that span, forexample, 30″ of the underside in one pass. The Sc-WSF detectors 227 aremounted on each side of the x-ray tube so as detect x-rays 236backscattered from vehicle 229. Power supplies, pulse and imageprocessors can be mounted appropriately. Chassis 234 of inspection unit230 on wheels 232 may be adapted to be maneuvered under vehicle 229 bymotor or manual control.

Mobile Transmission Inspection with L-Shaped Detector Array Segments

In accordance with another aspect of the present invention, a mobileinspection system, designated generally by numeral 240, is now describedwith reference to FIGS. 21A and 21B. A source of penetrating radiation(not shown, and described, herein, without limitation, in terms ofx-rays) is conveyed within a mobile inspection unit 241, which,typically, is capable of motion under its own power, although it mayalso be towed or otherwise transported, within the scope of the presentinvention. A beam 242 of penetrating radiation is emitted from mobileinspection unit 241, either as a swept pencil beam or as a fan beam, ineither case emitted in the plane designated as representing beam 242 inFIG. 21A. Inspected object 244, which may be a vehicle as shown, orotherwise (such as hauled cargo), traverses beam 242 during the courseof inspection, and, in the course of traversal, passes over an integralL-shaped detector unit 245, as now further described. Detector unit 245has a horizontal segment 246 and an upright segment 247, as indicated inFIG. 21B.

Each of the horizontal and upright segments 246 and 247 of L-shapeddetector unit 245 may be comprised of multiple parallel layers 249,providing for dual- or, more generally, multiple-, energy resolution ofdetected x-rays, so as to provide material identification, as describedabove with reference to FIG. 12. Additionally, upright detector arraysegment 247 may have multiple detector segments 248 in a directiontransverse to the direction of beam 242 and substantially along thedirection of relative motion between inspected object 244 and beam 242so as to provide an indication of skewness or lateral shift of thedetectors with respect to the beam, as described above with reference toFIGS. 14A-14C. Integral L-shaped detector unit 245 may be conveyed to asite of inspection aboard mobile inspection unit 241 or on a towed, orotherwise accompanying, trailer 250, and may be assembled, in part, upondeployment at the inspection site. Supplemental alignment aids, such asalignment laser 251, may be employed in establishing proper position andorientation of detector unit 245 relative to mobile inspection unit 241and beam 242.

Where examples presented herein involve specific combinations of methodacts or system elements, it should be understood that those acts andthose elements may be combined in other ways to accomplish the sameobjectives of x-ray detection. Additionally, single device features mayfulfill the requirements of separately recited elements of a claim. Theembodiments of the invention described herein are intended to be merelyexemplary; variations and modifications will be apparent to thoseskilled in the art. All such variations and modifications are intendedto be within the scope of the present invention as defined in anyappended claims.

What is claimed is:
 1. A method for discriminating among x-ray beams ofdistinct energy content, the method comprising: a. converting energy ofan x-ray beam incident upon a first volume of scintillation medium intoa first scintillation light; b. guiding light derived from, and at afirst wavelength longer than the first scintillation light via a firstplurality of wavelength-shifting optical waveguides; c. convertingenergy of x-ray radiation that has traversed the first volume into asecond scintillation light in a second volume of scintillation medium;d. detecting, with a first photodetector, photons at the firstwavelength guided by the first plurality of waveguides therebygenerating a first detector signal; and e. detecting, with a secondphotodetector, photons due to scintillation in the second volume ofscintillator material thereby generating a second signal; f. processingthe first signal and the second signal to provide a measure of a lowenergy component and a high-energy component of the x-ray beam incidentupon the first volume of scintillation material.
 2. A method accordingto claim 1, further comprising guiding light derived from, and at asecond wavelength longer than that of, the second scintillation lightvia a second plurality of wavelength-shifting optical waveguides to thesecond photodetector.
 3. A method according to claim 1, wherein thefirst volume of scintillation medium and the second volume ofscintillation medium are characterized by distinct spectralsensitivities to an x-ray beam traversing both the first and secondvolumes of scintillation medium.
 4. A method according to claim 1,wherein the second volume of scintillation medium is characterized ahigher sensitivity to photons exceeding a specified energy that thefirst volume of scintillation medium.
 5. A method according to claim 1,wherein the second volume of scintillator medium includes plasticscintillator.
 6. A method according to claim 1, wherein the first volumeof scintillator medium includes a lanthanide-doped barium mixed-halide.7. A method according to claim 1, further comprising: g. collectinglight produced by scintillation in a third volume of scintillationmedium traversed by the incident x-ray beam after transversal of thefirst volume of scintillation medium and before incidence upon thesecond volume of scintillation medium; h. coupling the light produced byscintillation in the third volume into coupled light within a waveguide;i. converting the light within the waveguide to a light of longerwavelength than the coupled light; and j. detecting the light of thelonger wavelength and producing a third signal for processing inconjunction with the first and second signals.
 8. A method according toclaim 1, further comprising enhancing a difference between low- andhigh-energy components of the incident x-ray beam by absorbing energyfrom the incident x-ray beam after transversal of the first volume ofscintillation medium and before incidence upon the second volume ofscintillation medium.
 9. A method according to claim 5, whereinabsorbing energy from the incident x-ray beam after transversal of thefirst volume of scintillation material includes traversal by the beam ofcopper intervening between the first volume of scintillation medium andthe second volume of scintillation medium.
 10. A method according toclaim 1, further comprising measuring an average atomic number ofmaterial traversed by an x-ray beam on the basis of the measure of thelow energy component and the high-energy component of the x-ray beamincident upon the first volume of scintillation medium.